GAZIANTEP, Turkey — The biggest surviving rebel stronghold in northern Syria is falling under the control of al-Qaeda-linked extremists amid a surge of rebel infighting that threatens to vanquish what is left of the moderate rebellion.

The ascent of the extremists in the northwestern province of Idlib coincides with a suspension of aid to moderate rebel groups by their international allies.

The commanders of five of the groups say they were told earlier this month by representatives of the United States, Saudi Arabia and Turkey that they would receive no further arms or ammunition until they unite to form a coherent front against the jihadists, a goal that has eluded the fractious rebels throughout the six years of fighting.

The freeze on supplies is unrelated to the change of power in Washington, where the Trump administration is engaged in a review of U.S. policy on Syria, U.S. officials say. It also does not signal a complete rupture of support for the rebels, who are continuing to receive salaries, say diplomats and rebel commanders.

Rather, the goal is to ensure that supplies do not fall into extremist hands, by putting pressure on the rebels to form a more efficient force, the rebel commanders say they have been told.

Instead it is the extremists who have closed ranks and turned against the U.S.-backed rebels, putting the al-Qaeda-linked groups with whom the moderates once uneasily coexisted effectively in charge of key swaths of territory in Idlib, the most important stronghold from which the rebels could have hoped to sustain a challenge to Syrian President Bashar al-Assad.

Moderate rebels still hold territory in southern Syria, in pockets around Damascus, and in parts of Aleppo province where they are fighting alongside Turkish troops against the Islamic State.

But the loss of Idlib to the extremists has the potential to prolong — or at least divert — the trajectory of the war at a time when the United Nations is reconvening peace talks in Geneva aimed at securing a political settlement. The talks opened Thursday with little sign that progress was likely.

The Syrian government and its ally Russia will now be able to justify intensifying airstrikes against the area, perhaps in alliance with the United States, which is already carrying out its own strikes against al-Qaeda targets in Idlib, analysts say.

“Idlib is now basically being abandoned to the jihadis. This might be the end of the opposition as understood by the opposition’s backers abroad,” said Aron Lund, a fellow with the Century Foundation. “They won’t have any reason to support it.”

The al-Qaeda-backed offensive appears to have been triggered by the Russian push last month to make peace with the same moderate rebel groups that the United States had in the past sought, unsuccessfully, to protect from Russian airstrikes. The al-Qaeda affiliate Jabhat Fatah al-Sham — which is still widely referred to by its previous name, Jabhat al-Nusra — has since led a series of raids, abductions and killings against moderate rebels, activists and Western-backed administrative councils across Idlib.

The most radical rebel groups have joined a new coalition created by Jabhat al-Nusra called Hayat Tahrir al-Sham. More moderate ones have sought protection by allying themselves with the largest non-al-Qaeda group, Ahrar al-Sham, which subscribes to a school of Salafist jihadism that is considered too extreme for the United States and its Western allies to countenance.

“Al-Qaeda is eating us,” said Zakaria Malahifji, an official with the U.S.-backed Fastaqim rebel group, explaining why his group has chosen to join with the Ahrar al-Sham alliance. “It’s a military alliance only, for protection from al-Qaeda,” he said. “Politically, we don’t share their views.”

Around a dozen U.S.-backed groups are still holding out against the pressure to join forces with the extremists, but they acknowledge that their cause is increasingly hopeless.

Radicals “are controlling every aspect of life, the mosques and the schools. They are radicalizing 14-year-old boys. Al-Qaeda ideology is spreading everywhere and we have been abandoned,” said Lt. Col. Ahmed Saoud, a ­Syrian army officer who defected and commands a rebel unit in the U.S.-backed Free Idlib Army, one of the groups that has stood aloof from the jihadists.

Suspending the supplies seems guaranteed only to ensure that al-Qaeda continues to expand, the rebel commanders say. “Of course if you cut off the moderate rebels, al-Qaeda will grow more powerful,” Malahifji said.

Under the three-year-old program initiated by the United States, rebel groups that have been vetted by the CIA receive support in the form of salaries, light arms and ammunition, and limited quantities of antitank missiles. The supplies are overseen by a military operations center known as the Musterek Operasyon Merkezi, or MOM, comprising representatives of the U.S.-backed Friends of Syria alliance.

But even if the supplies are restored, it is unclear whether the rebels will now be in any position to challenge al-Qaeda. One rebel group burned its stores of ammunition rather than let them be captured by Jabhat al-Nusra. Some supplies have already been captured. A video posted on YouTube this week by the new Nusra-led alliance showed its fighters destroying a government gun ­position using one of the U.S.-made TOW antitank missiles that were supplied to the moderate rebels, presumably seized by al-Qaeda allies.

Al-Qaeda-linked groups do not yet control the main border crossings into Syria from Turkey, but they control the access routes and towns and villages around them, enabling them to commandeer any supplies that come across, said Charles Lister of the Middle East Institute.

The al-Qaeda alliance now “has almost total control over what goes through the border,” Lister said. “There has to be more rebel unity before the international community can take the risk.”

The rebels now face an existential choice — to join the radical groups and risk being annihilated from the air by Russian and U.S. warplanes, or to unite to confront al-Qaeda and its allies and risk defeat on the ground by the better-armed and highly motivated Islamist militants.

Turkey, the rebels’ closest ally, is offering a third option: to leave the Idlib area entirely and head east to join the Turkish-backed operation, known as Euphrates Shield, underway against the Islamic State — a rival of al-Qaeda. As Turkey presses to convince the United States that it can muster a force strong enough to provide an alternative to the Syrian Kurds to participate in the battle for the Islamic State’s capital of Raqqa, it has been heavily recruiting support among the moderate rebels of Idlib.

But Idlib rebels do not want to surrender their territory to the jihadists to go fight on a different front, said Capt. Mohanned Junaid of Jaish al-Nasr, another U.S.-backed group that last week lost an estimated 69 members in a massacre of moderate rebels by one of the al-Qaeda affiliates.

“The whole of Idlib will be painted black, and that will give justification to the regime and Russian jets to bombard it,” he said.

Yet even with the moderate rebels confronting likely annihilation, feuds among them persist, precluding the alliance their international sponsors are seeking, said Saoud, the rebel commander. He is gloomy about the prospects for the rebels’ survival.

“If we don’t get any more support, we will just keep fighting each other and killing each other until we all are dead,” he said. “The regime will be watching us. This is what they want.”